a) Field of the Invention
This invention relates to a photosensitive element which has photolithographically viahole-forming ability and good adhesion to plated copper and is suited for use in the fabrication of a multilayer wiring board. This invention is also concerned with a process for the fabrication of a multilayer wiring board.
b) Description of the Related Art
Keeping step with high densification of printed circuit boards in recent years, there is an increasing demand for so-called multilayer printed boards provided with a plurality of circuit layers. For the fabrication of a multilayer printed circuit, it has heretofore been the practice to press plural laminates, each of which carries wiring formed in advance thereon, together with a thermoserving insulating sheet interposed between each two adjacent laminates. This conventional fabrication process however requires ultimate care upon its practice, because it involves problems such as alignment and shrinkage of substrates. Conventional multilayer printed boards therefore tend to have high cost. With a view to overcoming such problems, it has been proposed, instead of relying upon the laminate-pressing step, to form an insulating layer on a first conductive pattern layer and then a second conductive pattern layer on the insulating layer, that is, so-called stacked multilayer circuit boards have been proposed.
For the connection between layers in such a printed circuit board, a newly-developed interlayer connecting technique is adopted besides the conventionally-practiced, interlayer electrical connection making use of through-holes. The newly-developed interlayer connecting technique is a connecting technique of the non-through-hole type. According to this technique, holes are drilled in each insulating layer until the drills nearly extend through the insulating layer. As a further substitute, an excimer laser boring technique has been proposed recently.
These interlayer connecting techniques are of the non-through-hole types and holes are arranged only at locations needed between layers. It is hence possible to increase the freedom in the layout of wiring, thereby contributing toward a reduction in the number of layers in a multilayer circuit board and also high densification of a printed circuit board.
However, the interlayer connecting techniques which have been adopted to date fundamentally require successive formation of holes. The fabrication cost therefore increases with the number of holes. A multilayer board obviously requires many holes for the connection between layers. In high-density products, it is not rare that more than 10,000 holes are needed per board. The cost required for this drilling work is one of major causes for the high fabrication cost of multilayer wiring boards.
In meantime, so-called photolithographic viahole technique was proposed. According to this technique, all connecting holes are formed together at once by photolithography. This photolithographic viahole technique applies photolithographic processing, which is used primarily upon formation of conductive patterns (namely, a photosensitive resin layer is exposed and developed at desired locations to provide an etching resist or a plating resist), to interlayer insulating films. Because it photographically forms all holes at once, the fabrication cost is irrelevant to the number of holes. Further, small-diameter viaholes, whose formation is extremely difficult when drills are used, can be formed precisely. The photolithographic viahole technique is therefore considered to be most promising as an interlayer connecting technique. Multilayer printed boards with viaholes photolithographically formed using a photosensitive epoxy resin are disclosed, for example, in the article entitled "Characteristic Features and Application of Surface Layer Printed Circuit Board (SLC)" ("Electronic Materials", 103-108, April 1991) and also in Japanese Patent Application Laid-Open (Kokai) No. HEI 4-148590.
Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 4-180984 discloses examples of additive adhesive films containing an epoxy resin photoinitiator, that is, a photopolymerization initiator for epoxy resins. The additive adhesive films exemplified in this patent publication however do not contain any photocrosslinking agent for rubber so that no good image can be formed even when they are exposed to ultraviolet rays of high intensity.
In addition to the availability of a multilayer composite structure without any pressing step as described above, stacked multilayer circuit boards also have such advantages that small-diameter viaholes can be formed and a microcircuit can be formed. To progressively form circuitry by the stacking technique, however, it is indispensable to provide each conductive pattern by forming a conductor on an interlayer insulating film in accordance with an electroless plating process. Each conductive pattern so formed is also required to have sufficient adhesion with a substrate as expressed typically in terms of peel strength. Accordingly, an insulating material required as each interlayer insulating film for the stacking technique is needed to have formability of images such as viaholes, resistance to an electroless plating solution, and adhesion to plated copper.
Electroless plating solution resistant adhesives have been used for many years as additive adhesives and are composed primarily of a rubber material. For example, Japanese Patent Application hid-Open (Kokai) Nos. SHO 58-57776 and SHO 62-248291 disclose some examples composed primarily of an epoxy resin, a synthetic rubber and a phenol resin. In addition, Japanese Patent Application Laid-Open (Kokai) No. HEI 4-180984 discloses film-shaped adhesives which contain an epoxy resin, a synthetic rubber, a phenol resin, a chemical plating catalyst and a photosensitive aromatic onium salt. These illustrative adhesives show excellent roughening properties when treated with a roughening solution, and have good adhesion to plated copper, for example, peel strength higher than 2 kgf/m. However these materials do not have photosensitivity and, obviously, do not permit photolithographic formation of viaholes.
On the other hand, as heat-resistant resist materials, many examples are known as photosensitive solder masking materials or photoadditive materials. These illustrative materials are photosensitive materials inherently so that they are excellent in image forming ability and electroless plating solution resistance. They however have extremely poor adhesion to copper plated in an electroless manner. They are also accompanied by such a drawback that no sufficient roughening can be achieved even when treated by a roughening solution. Although they may have property as resists in an additive process, they are practically unusable as additive adhesives, namely, as interlayer insulating films upon photolithographic formation of viaholes.
The adhesives referred to above by way of example permit roughening when a special roughening solution is used, and show some improvements in the adhesion to plated copper. Their peel strength is however reported to be 1.0 Kg or less. Their adhesion is still insufficient especially in the outermost layer.
To be useful in stacked multilayer printed circuits, an insulating material is required to have photolithographic viahole forming ability, heat resistance, plated copper precipitating ability, and adhesion to plated copper. Under the circumstances, there is no photosensitive material that meets all the above requirements and can provide multilayer wiring boards with sufficient reliability.